Yes, if you are changing the format of your URLs from one extension to another, the correct thing to do is set a redirect from the old URLs to the new ones.
However, even if done correctly there can still be disruption to your current Google rankings, as the change is going to take time for Google to process & update and the fact that 301 redirects carry over most (but not 100%) of the ranking weight from the old URLs.
Here's a video from Matt Cutts here on the subject of page rank loss through 301 redirects.
If you don't want any fluctuations, stick with he URLs you have, if that is not an option then make sure to:
- Use correct 301 redirects
- Redirect URL-to-URL, for every page you want to keep.
- Don’t chain redirects
- Update all links through out the site, so they link to the new URLs
- Update your sitemap.xml file to reflect the new URLs